The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with calculating tissue perfusion using computed tomography (CT) scanners, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other modalities such as MRI, and is not limited to the aforementioned application.
Generally, CT scanners have a defined examination region or scan circle in which a patient, or subject being imaged is disposed on a patient couch. A fan beam of radiation is transmitted across the examination region from an radiation source, such as an x-ray tube, to an oppositely disposed array of radiation detectors. The x-ray tube and associated power supply and cooling components are rotated around the examination region while data is collected from the radiation detectors. Rotation of the radiation source is often achieved by mounting the radiation source to a rotating gantry which is rotated on a stationary gantry. For volume imaging, the patient couch is moved longitudinally. Continuous movement achieves spiral scanning whereas discrete steps achieve a series of parallel slices.
The sampled data is typically manipulated via appropriate reconstruction processors to generate an image representation of the subject which is displayed in a human-viewable form. Various hardware geometries have been utilized in this process. In third generation scanners, both the source and detectors rotate around the subject. In a fourth generation scanner, the x-ray source rotates and the detectors remain stationary. The detector array typically extends 360xc2x0 around the subject in a ring outside of the trajectory of the x-ray tube.
In a perfusion study, blood flow in tissues and vessels of interest is of primary concern. Typically, a contrast agent is injected into the subject and multiple xe2x80x9csnapshotsxe2x80x9d of the region of interest are taken over time. Present CT scanners are capable of taking 1 to 2 snapshots per second of the region, providing a series of images that tracks the contrast agent in near-real time.
One particular application of CT perfusion is helping to diagnose cerebral ischemia in patients who have suffered acute strokes. This type of study requires precise measurements over a period of time. One technique that is used in the calculation of perfusion is the maximum slope method, which calculates the maximum slope of a time vs. density curve and a maximum arterial enhancement. Perfusion is the maximum slope divided by the maximum arterial enhancement. Accuracy of the quantitative data is impacted by noise in the data, which may have several possible sources. These include patient motion, blood recirculation, partial volume effect, and other factors.
One method of reducing patient motion in a head CT scan, and thus improving the quality of the perfusion investigation, is immobilizing the head of the subject in an external restraint. Typically, such a device includes a strap that is connected to the patient couch that traverses the forehead of the subject, effectively eliminating head motion in a vertical direction (given that the subject is laying horizontally). However, the subject is still capable of movement laterally, as well as slight rotation of the head. These movements can seriously degrade the quality of a perfusion study, causing misalignment of the series of images, blurring a resultant image, and having adverse effects on the calculation of blood perfusion. The maximum density enhancement, measured in Hounsfield units (HU) can be reduced by 40% or more by motion that can occur despite the aid of a head restraint. The blurred images, and effects on perfusion measurements significantly impact the accuracy of quantitative measurements used in diagnosis.
Further, background noise is a factor that affects perfusion calculation, as well as the images associated therewith. Regions that exhibit low signal can be overshadowed by noise. In low blood flow regions, the maximum density enhancement and the noise can both be in the 2-4 Hounsfield unit range. Legitimate perfusion signals can be hidden decreasing the efficacy of the study as a whole. Filters meant to eliminate noise may also eliminate low strength perfusion signals effectively getting rid of good information along with useless information.
The present invention contemplates a new and improved method and apparatus which overcome the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of correcting for low signal is provided. A volume image of a region of interest is generated. Low signal voxels are identified and grouped into clusters. Perfusion values are determined for grouped and ungrouped voxels, and a single profusion image is generated.
In accordance with another aspect of the present invention, a method of improving a signal-to-noise ratio in provided. A perfusion study is performed, and time-density curves are generated for all voxels of a perfusion image. Low signal voxels are identified and grouped together. Representative time-density curves are found for each low signal voxel group, and function values are calculated therefrom. The results are applied to each member voxel of the group. The results are combined with results from normal signal voxels to produce a single perfusion image.
In accordance with another aspect of the present invention, a diagnostic imaging apparatus is provided. A signal analyzer monitors time-density curves of voxels of a volume image. A voxel binner groups low signal voxels. A voxel combiner combines intensity values of the voxels within the group. A perfusion calculator calculates perfusion values for the low and normal signal voxels.
One advantage of the present invention is a reduction of the negative effects of patient motion.
Another advantage resides in a reduction of the partial volume effect.
Another advantage resides in the reduction of the negative effects of blood recirculation.
Another advantage resides in the reduction of the effect of low amplitude signals.
Another advantage resides in the increased accuracy of curve fits.
Another advantage resides in reduction of errors caused by noise.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.